The present invention relates to a method for manufacturing a liquid crystal display device, and more particularly to a method for manufacturing a multi-layer liquid crystal display having multiple liquid crystal layers.
Conventional liquid crystal display (LCD) devices operate at low driving voltages, and, thus, consume relatively little electrical power. Accordingly, the LCD represents a significant development in technology and has been adopted in many applications. A polarized glass plate is typically used to control the amount of incident light applied to the LCD in conjunction with an active matrix including a simple X-Y matrix, or a thin film transistor (TFT) driving method. In other words, conventional twisted nematic (TN) type and super-twisted nematic (STN) type LCD devices require a polarized plate to control incident light. However, the polarized plate in the LCD intercepts more than 50% of the incident light while controlling light polarization, thereby reducing the efficiency of light use. For this reason, a background light source having a considerable brightness is required to obtain a picture image having acceptable brightness. Thus, in the case of a laptop word processor or computer which uses a dry cell battery or an accumulative battery cell as a power supply source, extended use is limited due to the excessive power consumption of the light source.
Also, in the conventional LCD including TN and STN matrix types, since liquid crystal is charged between two glass plates, it is necessary for a cell gap which is a light-controlled area to be strictly adjusted in order to form a uniform picture image. However, due to current technological limitations in the manufacturing of the glass plate, super-enlarging of an LCD panel is difficult to achieve.
Taking the above-described problems into consideration, to decrease the burden of cell gap adjustment, it is preferable that one sheet of glass substrate is used and that the polarized plate is not used to thereby increase efficiency in the use of light. Of course, LCDs without polarized plates do exist. Examples of such LCDs include a cholesteric nematic transition (CNT) type which uses a phase transition effect and a dynamic scattering mode (DSM) type which was devised during early LCD development. The DSM type LCD exhibits a slow response time and cannot be made thin, so that it is no longer in common use.
Also, another example of an LCD not using a polarized plate to increase the efficiency of light is a polymer-dispersed liquid crystal display (PDLCD). However, since the PDLCD is made of a polymer material more than half of whose volume is light-transmitting, the scattering of light should be brought about to obtain a clear contrast ratio. To attain these requirements, there is a structural limitation in that the thickness of the liquid crystal layer should be at least 20 .mu.m.
An LCD which adopts an electrical field effect type liquid crystal having a new structure in which the above conventional problems of the LCD are considerably improved, has been developed by Nobuyuki Yamamura and was filed on May 8, 1992 as Japanese patent application No. Hei 4-116146.
The above LCD has a fast driving speed and high light-utilization efficiency, in which the liquid crystal layer provided between the opposing electrodes is isolated by a plurality of insulation layers to form a multi-layer structure, the polarized plate is not used and only a single sheet of a glass substrate is applied.
A method for manufacturing a light-dispersion type liquid crystal display (LCD) device (devised by Nobuyuki Yamamura) is described hereinafter.
FIG. 1 is a schematic perspective view of the light-dispersion type LCD device and FIG. 2 is a plan-view of the LCD device as shown in FIG. 1. Referring to FIGS. 1 and 2, the LCD device has a plurality of field-effect type liquid crystal layers 11 between two opposing electrodes, i.e., first and second electrodes 2 and 8, with the liquid crystal layers supported by columns 13 located therebetween and acting as spacers; insulation layers 3 for separating liquid crystal layers 11 into a number of layers; and a plurality of liquid crystal inlet holes 10 for injecting the liquid crystal therethrough.
The thickness of each liquid crystal layer 11 is less than 3 .mu.m and the thickness of the insulation layer 3 is less than 5 .mu.m. In addition, the insulation layers 3 are generally made of light-transmitting epoxy resin or acrylic resin, but can be made of metal oxides, particularly, aluminum oxide.
The method for manufacturing a liquid crystal device is hereinafter described with reference to FIGS. 3A through 9B. Here, those figures having an "A" suffix are sectional views along A-A' of FIG. 2, while those having a "B" suffix are sectional views along B-B' of FIG. 2.
1) Indium tin oxide (ITO), which is a conductive material, is deposited on the surface of plastic or glass substrate 1. Then, a first electrode 2 having a predetermined pattern is formed by means of a photolithographic process (FIGS. 3A and 3B).
2) On the whole surface of substrate 1 on which first electrode 2 is formed, insulation layers 3 comprised of light-transmitting resin (for example, epoxy resin or acrylic resin) and dissolution layers 4 comprised of polyvinyl alcohol (PVA) are repetitively deposited using a spin coating method and a roll coating method, respectively (FIGS. 4A and 4B).
3) A second photolithographic process is performed for forming columns, which will be described hereinbelow, upon the top surface of the laminated structure. That is, photoresist 5 which is a light-sensitive resin is coated and then patterned to produce predetermined column-use holes 6. Thereafter, the portions which are not covered with photoresist are etched to thereby form column-use holes 6 (FIGS. 5A and 5B).
4) Photoresist 5 is removed. Then, column-use holes 6 are filled with light-transmitting insulation resin, and simultaneously, the insulation resin coats the whole surface of the laminated structure, so that columns 13 and insulation layers 7 are formed (FIGS. 6A and 6B).
5) ITO is deposited on insulation layer 7, so that second electrode 8 is formed opposing first electrode 2 on the upper surface of columns 13, by means of a photolithographic process (FIGS. 7A and 7B).
6) Photoresist is coated on the upper surface of the laminated structure. Then, a photolithographic process is performed so that the liquid crystal inlet holes 10 reach first electrode 2. Next, water, acetone or alcohol is injected through inlet holes 10, and thereby dissolution layer 4, i.e., PVA layer, is dissolved and removed. As a result, the liquid crystal inlet holes 10 and dissolution layers 4 become cavities 9, and insulation layers 3 support cavities 9 as columns (FIGS. 8A and 8B).
7) This partially manufactured liquid crystal device (LCD) is dried and then coated with liquid crystal in a vacuum. The pressure is then increased to the atmospheric pressure, whereby the liquid crystal permeates cavities 9 along liquid crystal inlet holes 10 so that a liquid crystal layer 11 is formed. When the filling of the liquid crystal is finished, a light transmitting insulation resin is coated on the whole surface of the laminated structure for sealing, to thereby form a protective layer 12. Accordingly, a desired product is obtained (FIGS. 9A and 9B).
However, in the manufacturing method, for forming the columns, a dry or wet etching should be performed in the second photolithographic process, and light-transmitting insulation resin should be coated, whereby the column-use holes are filled up.
At this time, after the filling is completed, the periphery of the column-use holes is stepped concavely by the influence of the holes, which can cause a degradation of step coverage (the ratio of coating on the sloped portion to that of the flat portion, when any kind of thin film is coated) in the second ITO deposition.
Therefore, after the formation of the second electrode (ITO electrode), when the liquid crystal inlet holes are formed and the dissolution layers are etched away, a wet etching should be performed for many hours. In this case, second electrodes formed around the column-use holes become severely swollen or lifted due to a peeling phenomenon.